CIHM 
Microfiche 


(Monographs) 


ICIMH 

Collection  de 
microfiches 
(monographies) 


Canadian  Inatituta  for  Hiatorical  Microraproductiona  /  Inatitut  Canadian  da  microraproductiona  hiatoriquaa 


if*K?«USC«  ftSSS 


Technical  and  Bibliographic  Notaa/Notas  tachniquas  at  bibliographiquaa 


Tha  Instltuta  has  attamptad  to  obtain  tha  bast 
original  copy  available  for  filming.  Faaturas  of  this 
copy  which  may  ba  bibliographically  unique, 
which  may  altar  any  of  tha  images  in  the 
reproduction,  or  which  may  significantly  change 
the  usual  method  of  filming,  are  checked  below. 


L'Institut  a  microfilma  la  meilleur  axamplaira 
qu'll  lui  a  eta  possible  da  se  procurer   Les  details 
de  cet  exemplaire  qui  sont  paut-Atre  uniques  du 
point  de  vue  bibliographiqua.  qui  peuvent  modifier 
une  image  reproduite.  ou  qui  peuvent  emger  une 
modification  dans  la  mAthode  normale  de  filmage 
sont  indiquAs  ci-dessous. 


0 


Coloured  covers/ 
Couverturc  de  couleur 


D 


Coloured  pages/ 
Pages  de  couleur 


Pyl    Covers  damaged/ 


Couverture  endommagAe 


E    Pages  damsged/ 
Pages  endommag^es 


□    Covers  restored  and/or  laminated/ 
Couverture  restaurAe  et/ou  pelliculAe 


□ 


Pages  restored  and/or  laminated/ 
Pages  restaur^es  et/ou  pelliculAes 


□    Cover  title  missing/ 
Le  tit 


itre  de  couverture  manque 


Pages  discoloured,  stained  or  foxed/ 
Pages  dAcolorAes.  tachatAes  ou  piquaas 


□    Coloured  maps/ 
Cartes  gAographiques  en  couleur 


D 


Pages  detached/ 
Pages  dAtachees 


n 


Coloured  ink  (i.e.  other  than  blue  or  black)/ 
Encre  de  couleur  (i.e.  autre  que  bieue  ou  noirel 


D 


Showthrough/ 
Transparence 


D 


Coloured  plates  and/or  illustrations/ 
Planches  et/ou  illustrations  en  couleur 


0    Quality  of  print  varies/ 
Qualite  in^gala  de  I'impression 


D 
D 


n 


Bound  with  other  material/ 
Relii  avec  d'autres  documents 

Tight  binding  may  cause  shadows  or  distortion 
along  interior  margin/ 

Lareliure  serree  peut  causer  de  I'ombre  ou  de  la 
distorsion  le  long  da  la  marge  intirieura 

Blank  leaves  added  during  restoration  may 
appear  within  tha  text.  Whenever  possible,  these 
have  bean  omitted  from  filming/ 
II  se  peut  que  certainas  pages  blenches  ajouttes 
lors  dune  restauration  apparaissant  dans  le  texte. 
mais.  torsque  cela  Atait  possible,  ces  pages  n'ont 
pas  M  filmAes. 


D 
D 

D 


Includes  supplementary  material/ 
Comprand  du  material  supplementaire 

Only  edition  available/ 
Seule  Edition  disponible 

Pages  wholly  or  partially  obscured  by  errata 
slips,  tissues,  etc..  have  been  refilmed  to 
ensure  the  best  possible  image/ 
Las  pages  totalament  ou  partiellement 
obscurcies  par  un  feuillet  d'errata.  une  pelure. 
etc..  cnt  it6  film^es  i  nouveau  de  facon  a 
obtenir  la  meilleure  image  possible 


Pagination  is  as   follows:    p.    [322]-333. 


0    Additional  comments:/  ,  .         .  r^^^-,    ^o-, 

Commentaires  supplAmentaires     ^^  pagination  est   comme   suit:    p.    [322] -333. 


This  item  is  filmed  at  the  reduction  ratio  checked  below/ 

Ce  document  est  filmA  au  taux  de  rMuction  indiquA  ci-dessous. 

10X  14X  18X  22X 


26X 


y 


30X 


J L 


12X 


1CX 


20X 


24X 


28X 


32X 


The  copy  filmed  here  has  been  reproduced  thanks 
to  the  generosity  of: 

Thomu  Fisher  Rare  Book  Library, 
University  of  Toronto  Library 

The  images  appearing  here  are  the  best  quality 
possible  considering  the  condition  and  legibility 
of  the  original  copy  and  in  keeping  with  the 
filming  contract  specifications. 


Original  copies  in  printed  paper  covers  are  filmed 
beginning  with  the  front  cover  and  ending  on 
the  last  page  with  a  printed  or  Illustrated  impres- 
sion, or  the  back  cover  when  appropriate.  All 
other  original  copies  are  filmed  beginning  on  the 
first  page  with  a  printed  or  illustrated  impres- 
sion, and  ending  on  the  last  page  with  a  printed 
or  illustrated  impression. 


The  last  recorded  frame  on  each  microfiche 
shall  contain  the  symbol  — ^  (meaning  "CON- 
TINUED">,  or  the  symbol  V  (meaning  "END"), 
whichever  applies. 

Maps,  plates,  charts,  etc.,  may  be  filmed  at 
different  reduction  ratios.  Those  too  large  to  be 
entirely  included  in  one  exposure  are  filmed 
beginning  in  the  upper  left  hand  corner,  left  to 
right  and  top  to  bottom,  as  many  frames  as 
required.  The  following  diagrams  illustrate  the 
method: 


L'exemplaire  filmA  fut  reproduit  grAce  A  la 
gAn^rositi  de: 

Thomas  Fisher  Rare  Book  Library, 
University  of  Toronto  Library 

Les  images  suivantes  ont  iti  reproduites  avec  le 
plus  grand  soin,  compte  tenu  de  la  condition  et 
de  la  nettet^  de  l'exemplaire  film*,  et  en 
conformity  avec  les  conditions  du  contrat  de 
filmage. 

Les  exemplaires  originaux  dont  la  couverture  en 
papier  est  imprim^e  sont  filmAs  en  commengant 
par  le  premier  plat  et  en  terminant  soit  par  la 
derniire  page  qui  comporte  une  empreinte 
d'impression  ou  d'illustration,  soit  par  le  second 
plat,  selon  le  cas.  Tous  les  autres  exemplaires 
originaux  sont  film^s  en  commen^ant  par  la 
premidre  page  qui  comporte  une  empreinte 
d'impression  ou  d'illustration  et  en  terminant  par 
la  dernidre  page  qui  comporte  une  telle 
empreinte. 

Un  des  symboles  suivants  apparaftra  sur  la 
dernidre  image  de  cheque  microfiche,  selon  le 
cas:  le  symbole  — ^  signifie  "A  SUIVRE",  le 
symbole  V  signifie  "FIN". 

Les  cartes,  planches,  tableaux,  etc.,  peuvent  Atre 
filmAs  i  des  taux  de  reduction  diffArents. 
Lorsque  le  document  est  trop  grand  pour  Atre 
reproduit  en  un  seul  clichA,  il  est  fiimA  i  partir 
de  Tangle  supArieur  gauche,  de  gauche  A  droite, 
et  de  haiit  en  bas,  en  prenant  le  nombre 
d'images  nAcessaire.  Les  diagrammes  suivants 
illustrent  la  mAthode. 


1 

2 

3 

1 

2 

3 

4 

5 

6 

v>i3i^tiKrtj%Kmm.  wntB" 


MICROCOPY  RESOLUTION  TEST  CHART 

NATIONAL  BUREAU  OF  STANDARDS 

STANDARD  REFERENCE  MATERIAL  1010a 

(ANSI  and  ISO  TEST  CHART  No.  2) 


A.B.Klugh. 


[From  'The  Auk,"  Vol.  XXI,  No.  3,  Julv,  i<>>4.1 


A    DISCUSSION    OF   THE   ORIGIN    OF    MI(;KATI(>N. 


HV    P.    A.    TAVKRNKR. 


■WV^mSSTk 


A    DISCUSSION    OF   THE   ORIGIN    Oi     MIGRATION. 

BY    1'.  A.  TWERNER. 

One  of  the  first,  if  not  the  very  first,  phenomena  of  animate 
nature  to  be  noticed  by  primeval  man,  must  have  been  that  of 
migration  ;  and  from  that  day  to  this  it  has  been,  to  a  greafer  or 
less  exient,  a  subject  of  great  interest  to  students.  In  the  present 
day  it  has  been  approached  from  many  different  sides,  and 
though  many  points  have  been  pretty  well  cleared  up,  others  are 
still  enveloped  in  a  haze  through  which  the  fundamental  princi- 
ples are  but  barely  visible,  while  others  still  remain  shrouded  in 
a  dense,  impenetrable  cloud  of  mystery. 

The  methods  by  which  birds  find  their  way  to  far  distant  points, 
the  manner  of  their  migrations,  etc.,  lie  without  the  scope  of  this 
paper,  and  will  not  be  referred  to  here.  Upon  these  points  we 
all  await  the  publication  of  the  results  of  the  investigations  now  in 
progress,  when  probably  many  obscure  points  will  be  cleared  up. 

Migration  consists  of  two  movements,  one  in  the  spring,  away 
from  the  winter  station  ;  and  the  other  in  the  f.ili,  towards  it  again. 
The  reason  of  the  latter  is  self-evident.  There  is  a  lack  of  food. 
If  they  did  not  return  in  the  fall  they  would  perish  of  hunger,  if 
not  of  cold.  From  general  observations,  it  seems  as  if  the  former 
had  a  larger  influence  than  the  latter,  and  it  is  the  northward 
movement  that  needs  explanation.  Why  should  a  bird  leave  a 
warm  land  of  plenty  to  journey  to  a  country  but  half  recovered 
from    the    frozen    embraces    of    an    arctic    climite  ?        It     seems 


i^'^'^'S.t^' 


S'^,v-7^v:'»^'-'''  ''^■> 


Vol.  XXI 


] 


Tavernkr,    Tht  Origin  of  Migration. 


323 


improbable  that  the  birds  themselves  realize  whj  they  migrate,  or 
what  benefits  are  to  be  thus  gained  or  enemies  escaped.  When 
the  proper  >eason  comes,  "the  spirit  moves  them,"  and  they  go  or 
come,  as  the  case  may  be.  However  instinctive  their  habit 
may  now  be,  there  must  have  been  a  time  when  migrations  were 
intelligent  movements,  intended  to  escape  some  danger  or  secure 
some  advantage ;  and  through  generations  of  repetition  they  have 
become  fixed  into  hereditary  habits,  closely  with  reproduction  and 
reproductive  seasons.  In  time  the  two  habits  became  so  inter- 
dependent that  the  awakening  of  the  sexual  desires  symp.ithetic- 
ally  affected  the  migratory  instincts  and  caused  restlessness  and 
a  desire  that  was  only  to  be  satisfied  by  the  accomplishment  of 
the  same  long  journey  that  their  progenitors  had  taken  for 
generations. 

Of  the  many  th  have  been  advanced  to  explain  this 

question,  I  will  n  that  seem  the  most  important  and 

the   most    genera  .vcv .     While    advancing   nothing   abso- 

lutely new,  I  wish  to  can  attention  to  one  factor  in  the  question 
that  has  not,  in  my  estimation,  been  given  its  due  importance, 
nor  has  it  been  recognized,  as  far  as  I  am  aware,  that  therein  lie 
possibilities  probably  capable  of  producing  all  the  phenomena  of 
migrations  as  we  now  see  them.     Of  this,  more  anon. 

There  is  a  theory  extant,  supported  by  W.  K.  Brooks  in  his 
'  Foundations  of  Zoology '  that  has  received  a  considerable 
amount  of  attention.  This  ascribes  migration  to  a  desire  to 
find  nesting  sites  secure  from  arboreal  Mammalia  and  Reptilia. 
This  supposes,  and  perhaps  correctly  so,  that  the  northern  nest- 
ing stations  are  safer  from  these  enemies  than  the  tropical  ones; 
though  any  one  familiar  with  our  northern  woods,  and  acquainted 
with  our  ubiquitous  red  squirrel,  may  have  good  grounds  for 
doubting  the  general  statement,  as  far  as  it  relates  to  mammals, 
at  least. 

There  are  certain  facts  of  distribution,  however,  that  this  theory- 
fails  to  explain,  and  which  seem,  indeed,  to  be  in  direct  antago- 
nism to  it.  Typical  instances  of  this  can  be  seen  in  the  distribu- 
tion and  ranges  of  the  families  of  Cuckoos  and  Doves.  Also  the 
occurrence  of  such  an  elaborate  md  careful  nest  builder  as  the 
Baltimore  Oriole,  as  far  north  as  the  Transition  fauna.     Surely, 


3^4 


TaveRNKR,    The  Origin  of  MigraiioH. 


TAak 


such  a  nest  as  this  bird  builds  would  be  as  secure  from  these 
enemies  in  the  heart  of  the  tropical  forests  as  in  the  temperate 
one«.  Therefore,  safe  nesting  sites  could  not  be  the  object  of 
their  migrating—  unless  the  peculiar  form  of  nest  was  evolved 
after  the  migratory  habit  had  been  formed.  This,  however,  d>  s 
not  seem  to  have  been  the  case.  Such  a  likeness  is  exhibited  m 
the  forms  of  the  nests  throughout  the  whole  family,  that  we  are 
forced  to  conclude  that  this  type  of  nest  was  used  by  the  cnmmon 
ancestor  of  Idcrus,  which  must  have  been  before  the  Baltimore 
Oriole  became  migratory. 

The  cuckoos  and  doves  above  mentioned,  are  notoriously  care- 
less nesters,  and  under  this  hypothesis,  we  would  expect  that 
migration  would  have  been  forced  upon  the  whole  of  these  fami- 
lies, or  at  least  upon  a  considerable  number  of  the  species  com- 
posing them.  Contrary  to  this,  we  find  that  these  are  peculiarly 
tropical  and  subtropical  families,  and  but  a  very  small  percentage 
of  them  ever  get  up  into  northern  latitudes. 

It  may  be  held  that  the  above  cases  are  exceptions,  caused  by 
varying  local  conditions,  but  it  still  remains  to  bo  proved  that  the 
generality  of  tropical  nesters  take  any  greater  nesting  precautions 
than  northern  ones  of  the  same  class,  as  would  assuredly  be  the 
case  if  the  above  were  the  correct  solution  of  the  problem. 
Furthermore,  there  are  grave  reasons,  to  which  I  will  refer  later, 
for  doubting  that  inadequate  nesting  habits  could  ever  be  the 
cause  of  migrations. 

A  second  theory,  advanced  under  the  auspices  of  Mr.  Chas. 
Dixon,  refers  the  movement  to  a  natural  desire  of  the  individuals 
of  a  species  to  disperse  during  the  breeding  season,  and  draws 
attention  to  the  fact  that  the  bird  population  is  more  scattered 
during  the  breeding  season  than  at  other  times.  He  utterly 
refutes  the  idea  that  adverse  circumstances  of  either  food,  tem- 
perature, or  enemies  can  force  a  bird  to  change  its  range,  and 
cites  instances  of  the  Great  Auk,  Labrador  Duck,  and  other  spe- 
cies that  have  suffered  extermination  rather  than  forsake  their 
accustomed  habitat.  Mr.  Dixon  evidently  regards  this  dispersal 
as  effecting  a  reduction  in  the  density  of  the  population.  It 
certainly  does  result  in  this  among  the  adult  inhabitants,  but  it 
is  open  to   question  if  we  assume   that  the  total  population  is 


._'U 


Vol.  XXI 
1904 


] 


T.WKRNKH.    Ike  Oritrin  of  Migration. 


.125 


thus  affected.  His  conclusion  is  apparently  based  ujwn  the  well 
known  and  indisputable  fact  that  birds  are  harder  to  find  during 
the  breeding;  season  than  at  other  t\ines.  It  must,  howevi-r,  be 
remembered  that  for  each  pair  of  breeding  birds  observed,  there  is 
somewhere  about  a  nest  full  of  young  that  are  not  seen  at  all. 
These  young  are  of  as  much  economic  importance  in  reckoning 
population  as  the  adults,  and  as  such  must  be  taken  into  consid- 
eration. On  the  whole,  I  doubt  very  much  whether  the  bird  pop- 
ulation in  the  oreeding  season  is  any  less  per  given  unit  of  territory 
than  at  other  times. 

That  migration  is  caused  by  a  natural  dispersal  of  the  adults 
during  the  breeding  season  must  be  admitted.  But  this  is  beg- 
ging t  question.  Migration  is  a  dispersal ;  and  conversely,  this 
dispers.i  ,  as  't  manifests  iiself,  is  migration.  The  author  fails  to 
explain  the  cause  of  the  natural  dispersal.  The  object  of  this 
scattering  may  be  seclusion,  either  for  privacy  or  safety.  If  for 
privacy,  it  seems  to  defeat  its  own  ends  when  such  birds  as  the 
herons,  swallows,  and  like  gregarious  nesters  congregate  in  great 
communities  to  perform  their  marital  duties.  If  safety  is  sought, 
it  presupposes  that  all  the  safe  nesting  sites  are  monopolized  by 
other  species  and  the  migrants  are  crowded  out. 

In  our  own  country,  we  can  readily  see  that  but  an  infinitesimal 
fraction  of  possible  sites  are  thus  occupieH.  How  rare  it  is  for  a 
nesting  place  to  be  used  a  second  time  L^  iififerent  individuals, — 
except  in  the  case  of  woodpeckers'  holes,  where  it  is  obvious  that 
the  supply  is  limited, —  any  field  worker  knows.  If  desirable 
forked  branches,  etc.,  were  at  such  a  high  premium,  this  would 
occur  frequently.  If,  then,  the  above  is  true  in  our  own  country, 
how  much  more  must  it  be  true  in  the  tropical  stations,  where, 
though  the  population  of  both  birds  and  their  enemies  is  greatly 
increased,  the  luxuriant  vegetation  affords  an  mfinitely  greater 
number  of  desirable  sites  for  nesting.  Crowding  in  this  sense 
seems  impossible. 

That  individual  birds  cannot  be  driven  from  what  they  regard 
as  their  proper  stations,  may  possibly  be  admitted ;  but  that  spe- 
cies cannot  (when  the  adverse  changes  in  surroundings  take  place 
gradunlly  enough),  is  absurd.  As  far  as  I  am  aware,  there  are 
three  principal  ways  by  which  geographical  distribution  can  be 


'SL2^ 


jit»'%' 


\ji^'^ '>v;i\r^' 


3^6 


TwKRNeR.   Tke  Origin  of  Migration. 


tAuk 
July 


effected.  One  is  the  sudden  irruption  of  a  species,  when  it  sud- 
denly appears  in  numbers  in  a  territory  where  it  had  been  either 
extremely  rare,  or  entirely  absent.  Kxamples  of  this  are  to  be 
seen  in  the  sudden  occurrence  of  the  Sand  (Irouse  in  Furope  in 
1888;  the  appearance  of  great  flocks  of  Brunnith's  Murre  on 
Lakes  Ontario  and  Erie,  1894-97.  and  the  great  movements 
occasionally  noted  in  Lemmings.  Of  the  underlying  causes  of 
these  strange  migrations,  whether  they  are  due  to  inner  psycho- 
logical or  outer  physical  phenomena,  we  are  ignorant.  These 
strange  overflows  seem  so  erratic  and  abnormal  in  the  light  that 
invasions  of  this  kind  do  not  succeed  in  forming  permanent  settle- 
ments on  the  new  grounds,  that  it  would  be  reckless  at  present,  to 
use  them  as  a  basis  for  theorizing,  until  all  other  means  fail. 

The  second  method  is  by  a  force  exer;<.-d  from  within  an  estab- 
lished range ;  and  the  third,  an  attractive  one  acting  from  with- 
out. These  two,  however  antagonistic  as  they  may  superficially 
seem,  are.  at  root,  one  and  the  same.  They  are  both  caused  by 
differences  in  the  desirabi'ity  of  two  stations.  One  is  caused  by 
a  decrease  in  the  desirability  of  a  present,  and  the  other  by  an 
increase  of  the  same  quality  in  an  adjoining  ter  "^ory.  They  are 
but  ratios  of  desirability,  and  can  both  '  e  expressed  by  fractions 
whose  values  depend  upon  the  relative,  not  the  numerical  size  of 
their  terms.  If,  then,  attraction  is  but  a  phase  of  driving,  and 
birds  cannot  be  driver,  from  their  haunts,  we  are  forced  to  discard 
all  our  present  theories  of  geographical  distribution  and  return  to 
that  of  special  creation,  or  found  our  science  upon  the  unknown 
quantities  of  general  irruption  probably  caused  by  psychological 
disturbances  of  whose  origin  and  intent  we  are  ignorant. 

Ranges  can  be,  have  been,  and  in  the  course  of  time,  must  many 
times  have  been,  changed  by  necessity  when  the  changes  in  con- 
ditions occur  slowly  enough  so  that,  though  individuals  may  not, 
the  whole  species  might  have  advanced  or  retreated.  In  this 
same  manner,  we  know  that  even  our  forests  have  migrated  back 
and  forth  across  the  continent  before  the  face  of  the  glacial  ice, 
climbed  mountains  and  descended  valleys,  though  each  individual 
tree  or  plant  remained  rooted  for  life  to  the  spot  where  it  origi- 
nally sprouted.  If  plants  can  and  have  done  this.  I  see  no  reason 
why  birds  could  not  also,  as  ev^  the  most  extreme  case  of  local 


^ 


Vol.  XXI 
1004 


] 


TavkkNKR,    Th4  Origin  »/  MignttioH 


.V7 


attachment,  a  bird  can  never  be  as  tirmly  tixed  to  its  station  as 
trees  and  plants  are  to  th.irs. 

The  cases  of  extermination  cited  art  •  ner<  .he  chanj^es  had 
come  too  suddenly,  or  where  the  species  hau  oecome  stereotyped 
or  indexible  in  habits  and  structure  by  too  long  and  great  success 
under  peculiar  conditions,  and  so  lacked  the  elasticity  of  nature 
necessary  to  modify  itself  and  its  life  to  slight  changes  of 
environment. 

A.  R.  Wallace  has  outlined  another  idea  on  the  subject,  lie 
suggests,  in  '  Island  Life,*  that  the  migrants  are  in  search  of  soft- 
bodied  insects  suitable  for  nestlings  ;  that,  as  the  season  .-.dvances 
in  the  tropics,  it  becomes  dryer  and  dryer,  and  such  inserts  soon 
disappear.  According  to  this  view,  it  seems  at  tirst  sighi  .  be  a 
Seeking  after  food  of  a  certain  quality.  Reduced  to  its  lowest 
terms  however,  it  appears  as  a  very  different  matter,  nanvly.  a 
question  of  quantity.  It  is  admitted  that,  even  in  the  tropic-., 
there  is  at  least  a  short  season  when  there  are  insec'  u*  a  suitab.  ■ 
quality  for  nestlings.  That  th-s  season  is  long  ei  ,h  to  raise 
birds,  is  evident,  for  many  species  closely  related  to  our  migrants 
successfully  nest  and  raise  their  broods  there.  If  all  birds  bred 
there  at  this  same  period,  there  would  be  suitable  food  there  and 
migrations  would  be  unnecessary.  That  they  do  not,  is  an  indica- 
tion that  some  other  factor  enters  into  the  question,  and  it  seems 
very  probable  that  all  birds  breeding  contemporaneously  would 
exhaust  the  supply  of  such  food.  The  question,  then,  is  one  of 
quantity  more  than  quality. 

It  rn.iy  be  objected  that  each  species  requires  its  own  special 
food  .u  the  critical  nesting  period,  which  may  not  be  obtain  .ole 
everywhere.  Now,  if  there  is  any  truth  in  our  present  evolution- 
ary theory,  great  changes  in  food  habits  have  occurred  in  all  our 
spec:  !s.  But  the  new  food  supply  must,  in  each  and  every  case, 
have  occurred  before  the  habits  and  structure  for  utilizing  it 
appeared.  Therefore,  food  habits  could  never  have  originated 
migrations,  though  migration  undoubtedly  has  had  a  great  influ- 
ence in  modifying  food  habits. 

It    must   be   remembered  also,  that  migration  is  a  dangerous 
undertaking  to  a  race.     A  journey  covering  thousands  of  niik 
to  be  performed  against  innumerable  enemies,  both  personal  and 


328 


Taverner,   The  Origin  of  Migration. 


PAuk 


elementary,  into  a  country  just  recovering  from  the  rigors  of  winter, 
is  a  very  hazardous  solution  of  any  problem.  Especially  must 
this  have  been  true  in  the  early  days  of  the  habit,  when  the  races 
were  much  less  adequately  provided  with  hereditary  experience 
and  structure  necessary  for  its  successful  conclusion.  In  this 
light,  it  seems  highly  improbable  that  anything  short  of  the  stern- 
est necessity  would  favor  the  development  of  a  habit  so  fraught 
with  danger  to  the  individuals  of  a  species ;  and  that,  if  any  less 
hazardous  solution  were  possible,  it  would  have  been  taken  advan- 
tage of. 

The  great  diversity  of  food  and  nesting  habits  exhibited  by 
closely  allied  species,  shows  how  easily,  comparatively  speaking, 
these  habits  are  modified.  Therefore,  if  any  peculiar  nesting  or 
food  requirements  menaced  the  welfare  of  tropical  residents  to  the 
extent  that  must  have  been  necessary  to  produce  migration,  it  is 
reasonable  to  suppose  these  habits  would  have  been  altered  to 
suit  surroundings  long  before  such  a  dangerous  habit  as  migra- 
tion could  have  been  adopted. 

The  natural  inference  is  that  the  problem  was  something  that 
could  be  solved  in  no  less  hazardous  way.  For  it  would  be  much 
easier  for  birds  to  learn  to  build  woven  pensile  nests  at  the  end  of 
long  slender  branches,  or  to  adopt  food  that  closely  allied  species 
found  acceptable,  than  to  create  all  the  elaborate  instincts,  powers 
and  structures  necessary  to  enable  them  to  traverse  great  stretches 
of  country  unguided,  and  in  the  face  of  meteorological  disturb- 
ances, new  enemies,  strange  foods,  and  all  the  dangers  attendant 
upon  migration.  These  grounds,  then,  alone  seem  sufficient  to 
discredit  any  such  phenomena  as  the  foregoing,  as  prime  causes 
in  the  origination  of  this  habit. 

The  one  cause  that  seems  adequate  to  produce  such  great 
results,  is  that  one  which  ultimately  rules  the  whole  animate  world 
—  the  sufficiency  of  the  food  supply.  Admitting  that  in  the  trop- 
ics there  is.  at  any  time,  or  more  especially  during  the  migration 
seasons,  a  lack  of.  or  a  severe  struggle  for  food,  and  we  have  a 
necessity  sufficiently  imperative  to  cause  the  origin  of  any  habit 
that  it  is  possible  to  form.  Mr.  J.  A.  Allen,  and  others,  have 
shown  that  the  usual  struggle  for  existence,  always  and  every- 
where intensely  severe,  is  sufficient  to  cause  an  overflow  into  an 


Vol.  XXI 


] 


Tavf.rnkr.    Tin-   Ofis^in  of  Migiati.m. 


y-^ 


adjoining  area  whenever  that  area  assumes  conditions  favorable 
for  the  support  of  an  increased  population.  The  return  of  spring 
causes  the  favorable  conditions  in  the  north,  and  the  spring  migra- 
tion is  the  evidence  of  the  overflow.  The  approach  of  winter  influ- 
ences life  in  the  same  manner,  but  the  overflow,  or  migration  is  in 
the  opposite  direction. 

Mr.  Allen  has  very  aptly  applied  the  saying  that  "  Nature  abhors 
1  vacuum."  and  suggests  that  migration  is  the  only  manner  in 
which  a  zoological  vacuum,  in  a  country  whose  life-supjiorting 
capacity  is  a  regularly  fluctuating  quantity,  can  be  tilled  l)y  non- 
hibernating  animals. 

That  this  view  is  correct,  I  do  not  think  can  be  doubted,  but 
there  is  another  factor  in  the  case  that  does  not  seem  to  have 
been  generally  perceived.— a  fact  that  strengthens  the  foregoing 
rea.soning  manifold.  True.  Mr.  Newton,  in  his  'Dictionary  of 
Birds  '  has  suggested  it,  but  without  apparently  perceiving  what 
a  powerful  factor  it  must  prove  in  the  case.  I  refer  to  the  effect 
of  the  large  increase  of  life  in  the  breeding  season,  in  an  already 
thickly  populated  count:  ,,  such  as  the  southern  stations  must  be 
just  previous  to  the  spring  migration,  coincident  wuh  the  opening 
up  for  settlement  of  a  vast  adjoining  and  practically  unoccupied 
territory,  by  the  seasonal  recession  of  the  winter  ice  cap.  I'nder 
the  "Law  of  Malthus"  we  find  a  country  to  the  south  of  us,  popu- 
lated to  its  fullest  extent  during  the  winter.  .Spring  comes,  and 
nearly  every  pair  of  birds  has  a  nest  full  of  young,  requiring 
great  quantities  of  food.  The  food  demand  must  be  increased  to 
many  times  what  it  was  before.  There  would,  of  course,  be  an 
increase  in  this  food  supply,  due  to  the  influence  of  spring,  but  it 
would  not  he  in  proportion  to  the  demand.  This  inadequacy  of 
the  food  supply  is  brought  home  to  us  very  clearly  if  we  reflect 
upon  the  fact  that  it  takes  the  whole  northern  hemisphere  to  sup- 
port the  species  in  the  summer  that  all  through  the  winter  were 
confined  to  a  very  limited  territory  :  and  that  even  then,  during 
the  time  of  greate.st  dispersal  and  food  supply,  the  competition  is 
always  keen.  Considering,  then,  that  this  great  increase  in  jjopu- 
lation  iiappens  contemporaneously  with  an  equal  growth  of  the 
food  producing  territory  due  to  the  return  of  spring,  it  does  not 
seem  at  all  wonderful  that  the  birds  should  migrate  to  utilize  a 


330 


Taverner,    The  Origin  of  Migration. 


TAuk 
Ljuiy 


plentiful  food  supply  and  escape  death  by  the  causes  attendant 
upon  the  evils  of  insufficient  nourishment. 

Migration,  if  the  outcome  of  these  phenomena,  probably  would 
have  originated  in  the  following  manner.  In  the  beginning  of  the 
breeding  season,  the  competition  would  originate  in  the  areas 
containing  the  earliest  breeders,  and  would  be  severest  in  the 
most  productive  districts.  Here  the  strongest  species  would  soon 
drive  out  the  weaker  ones  and  the  later  breeders,  which,  having 
no  parental  ties  to  bind  them  to  any  one  locality,  would  be  more 
easily  forced  to  leave  than  those  already  possessing  nests  —  all 
other  things,  of  course,  being  equal.  These  species,  driven  away, 
would  encroach  on  others,  forcing  them  out,  in  their  turn,  to  tres- 
pass upon  a  wider  circle  of  species.  Thus  the  pressure  arising 
from  the  congestion  originating  probably  in  the  center  of  the  win- 
ter residential  area,  would  be  felt  to  the  farthest  points  of  the 
populated  territory.  Any  stringency  of  food  supply  invariably 
causes  greater  exertions  on  the  part  of  the  inhabitants,  and  hence 
wider  ranging;  and  the  slightest  increase  in  sustaining  power  of 
adjoining  lands  would  be  immediately  found  and  taken  advantage 
of.  As  these  species  moved  into  the  new  country,  their  places 
would  be  quickly  taken  by  those  behind,  and  as  the  congestion 
was  relieved,  the  impelling  force  would  be  constantly  reinforced 
by  the  nesting  of  the  later  breeders  as  the  season  progressed. 

The  increase  of  population  and  life-supporting  area  would  pro- 
ceed regularly  and  evenly,  so  that  the  pressure  would  never  exceed 
the  relief.  This  nice  balance  would,  of  course,  have  been  secured 
according  to  the  laws  of  survival  of  the  fittest  —  undesirable  forms 
that  would  disturb  the  equilibrium,  being  either  modified  or  elimi- 
nated. 

Thus  each  species,  crowded  on  by  those  behind,  and  enticed  by 
the  advance  of  those  in  front,  would  proceed  onward  until  their 
own  particular  station  had  been  reached.  This  point  would  be 
determined  by  one  or  more  of  several  factors.  The  most  obvious 
of  these  would  be  the  failure  of  their  particular  food,  the  arrival 
of  tlieir  ne;,ting  season,  and  the  absence  of  superior  competitors. 
When  a  species  had  reached  this  stage  in  its  own  particular  migra- 
tion, it  would  settle  down  and  nest,  and  from  then,  to  the  end  of 
its  nidificalioi)  periud.   would  be  tixed,  and  by  its  own  increase 


Vol.  XXI 
l()04 


] 


T.WERNER,    The  Origin  of  Migration. 


33 » 


I 


aid  in  driving  forward  those  that  had  not  yet  found  suitable  con- 
ditions for  nesting. 

In  the  incipiency  of  the  migration  habit,  the  individual  move- 
ments would  be  small,  perhaps  originating  in  a  pair  of  birds 
discovering  an  unexpected  store  of  food  on  the  side  of  a  hill 
opposite  their  usual  haunts.  The  birds  that  were  bred  here 
would  find  their  way  back  the  next  year  with  greater  ease  than 
their  parents  did  originally,  and  would  be  in  a  position  to  make 
further  advances  to  the  hill  beyond.  So  each  year,  as  the  glacial 
ice  receded,  the  territory  suited  for  summer  occupancy  would  be 
slightly  enlarged,  and  the  birds  would  each  succeeding  year, 
during  the  period  of  greatest  stress,  find  sustenance  a  little  to  the 
northward  of  the  preceding  season's  uttermost  range. 

The  migratory  movements  and  the  differentiations  of  the 
breeding  season  are  so  closely  connected  that  it  is  difficult  to 
determine  which  originated  first.  Migration  would  delay  breeding 
in  the  species  that  showed  the  slightest  inclination  towards  the 
habit ;  and  conversely,  a  delayed  breeding  season  would  actively 
assist  the  evolution  of  migration.  The  origination  of  both  may 
have  been  simultaneous,  though  it  is  hard  to  ima<rine  a  time  when 
some  slight  traces  of  migration  would  not  have  been  beneficial  to 
the  races.  At  any  rate,  their  effects  would  have  been  cumulative, 
each  increasing  and  fixing  the  others.  Once  started,  then,  either 
or  both  would  be  rendered  more  and  more  pronounced,  through 
natural  selection,  until  the  extreme  limit  profitable  for  each 
species  was  reached. 

The  gradual  extension  of  the  extreme  summer  range,  as  the 
glacial  ice  cap  retreated,  would  most  probably  have  been  by 
means  of  the  younger  individuals,  or  birds  in  their  first  breeding 
season,  of  each  species,  as  these  would  be  weaker,  and  more  easily 
driven  than  the  older  ones  that  would  have  become  more  attached 
to  their  local  habitats.  It  seems  universally  true  that  young 
birds  do  not  often  return  to  breed  in  the  immediate  vicinity  of  the 
place  where  they  are  raised.  There  is  a  dispersing  influence  of 
some  sort  at  work  here.  It  is  said  that  the  older  ones  drive  their 
offspring  away  from  their  hunting  grounds  when  those  offspring 
are  able  to  take  care  of  themselves.  I  cannot  say  from  actual 
experience   that  they  do  this,  but  it  .seems  .so  advantageous  a 


332 


Taverner,    The  Origin  of  Migration. 


r.Au 
LJu 


uk 

ly 


habit  that  its  development  is  not  only  possible  but  very  probable, 
and  just  what  the  student  of  evolution  would  expect. 

This  scattering  of  the  younger  individuals,  however  it  was 
brought  about,  would  then  favor  the  extension  of  the  migration 
range  by  tiie  ones  thus  driven  to  wander  from  their  accustomed 
haunts.  As  further  substantiation  of  this,  it  is  to  be  noticed  that 
birds  found  far  from  their  natural  haunts  are  usually  immature 
specimens. 

A  young  bird  on  its  tirst  spring  migration,  would  naturally 
return  to  the  famili.ir  place  where  it  was  raised.  Being  driven 
away  from  here,  it  would  wander  about  until  it  found  a  suitable 
location  for  its  own  breeding —  perhaps  a  mile,  perhaps  two,  may- 
be less,  away  from  its  original  home.  The  succeeding  years, 
it  woukl  return  t'  this  new  haunt,  and  the  range  of  the  species 
could  be  extended  by  its  offspring.  Thus,  each  bird  would  follow 
the  route  taken  by  its  parents,  and  thus  each  point  on  a  migration 
route  would  indicate  the  place  that  was  once  the  ultimate  goal  of 
the  migrations  of  its  ancestors. 

Migrations  to  true  oceanic  islands  are  more  difticult  to  explain 
along  these  lines,  but  I  do  not  think  that  they  invalidate  the 
reasoning  in  any  way.  Mij:!ating  birds  certainly  have  wonderful, 
and  as  yet  mysterious,  senses  of  location  and  direction,  and  it  is 
not  too  much  to  say  that  a  bird,  once  it  has  traveled  a  certain 
journey,  is  usually  able  to  find  its  way  over  the  same  path  again. 
A  p.iir  of  birds  have  only  to  be  storm-blown  to  one  of  these 
isolated  spots,  breed  there,  and  return  with  its  progeny,  to  start  a 
tendency  in  their  offspring  to  migrate  to  the  same  place  again. 
As  long  as  tlie  least  tendency  to  an  advantageous  migration  were 
started,  natural  selection  would  confirm,  increase,  and  fix  the 
hal)it  firmly;  and  a'ong  with  this,  the  new  senses,  structures  and 
habits  necessary  to  their  accomplishment.  It  is  unlikely,  however, 
that  this  tyi)e  of  migr.ition  could  be  started  until  after  certain 
powers  and  senses  had  been  developed  by  migr.itions  to  other 
localities.  They  must,  therefore,  be  regarded  as  secondary  move- 
ments originally,  though  in  some  cases  they  have  become  now  the 
prime  or  only  migrations  of  the  species  by  the  extermination  of  all 
those  individuals  that  adhered  to  the  original  routes. 

The  return  movement  in  the  fall  i'j  the  same  thing,  nearlv,  as 


Vol.  XXI 

1(;04 


] 


T.WKRNER,    The   Origin  of  Mitrrutivn. 


333 


the  spring  migration,  but  reversed.  The  shortage  in  food,  how- 
ever, is  not  caused,  except  indirectly,  when  he  first  migrai 
encroach  upon  those  below  them,  by  the  increase  of  population, 
but  by  the  direct  failure  of  the  food  supply.  It  is  perfectly 
evident  that  certain  species  must  return  south  again,  or  stay  and 
surely  starve.  The  total  population,  however,  of  any  area,  cannot 
permanently  remain  greater  than  the  number  that  can  be  sustained 
through  the  season  of  least  food  supply.  During  the  height  of 
the  breeding  season,  there  are  many  more  birds  than  can  be 
carried  through  the  winter  in  the  restricted  southern  stations,  And 
if  they  are  to  return  there  again,  the  excess  must  be  got  rid  of. 
Many  of  them  are  killed  off  at  a  very  tender  age  — probably  the 
great  majority  of  them  fail  to  survive  the  fledgling  stage.  Many 
more,  young  and  inexperienced,  must  perish  when  first  they  leave 
the  protecting  influence  of  the  parent's  care.  Others  are  bat- 
tered about  by  the  storms  and  de.stroyed  by  the  perils  incident  to 
the  fall  migration.  The  few  surplus  that  remain  are  subjected  to 
a  stricter  and  stricter  process  of  selection  as  they  reach  more  con- 
gested areas ;  and,  in  the  end,  the  total  population  fits  into  its 
place  in  the  winter  quarters,  to  the  extreme  limit  of  the  sup- 
porting powers  of  the  land. 

These  migrations,  in  their  earliest  stages,  must  then  have 
originated  in  a  conscious  seeking  for  food  — not  special  food,  but 
any  food  that  would  support  them.  Accidental  wanderings  taught 
them  where  to  find  it,  and  experience  suggested  their  return  there 
on  the  first  approach  of  a  stringency  in  the  food  supplies.  In 
course  of  time,  the  movement  became  habitual,  and  generations 
of  repetition  rendered  it  instinctive.  Instinct,  having  the  same 
relation  to  judgment  as  automatic  machinery  has  to  ordinary 
mechanism,  would  be  favored  through  natural  selection  ;  and  as 
the  birds  acquired  the  peculiar  pouers  necessary,  migrations 
assumed  all  the  varied  phenomena  that  they  exhibit  to-day. 


